In a hydraulic steering apparatus of a work vehicle, including a work machine such as a bucket, a variable displacement steering pump is generally used so as to decrease the pressure loss. In recent years, a load-sensing type apparatus, for controlling the displacement of such a steering pump, has been employed as an answer to the request of saving energy. In the load-sensing type apparatus, an excessive hydraulic torque is avoided in the manner that the steering pump discharges an oil quantity nearly equal to the oil quantity applied to a steering cylinder in accordance with the rotational speed of a steering handle.
The first background example (a load-sensing type apparatus for controlling the displacement of a steering pump) will be described with reference to FIG. 6. The displacement V (the discharge per revolution) of a variable displacement steering pump 2, driven by an engine 1, is controlled by displacement control means 5, comprising a servo cylinder 3 and a load-sensing valve 4. A steering cylinder 6, for driving the steering system of a work vehicle, is controlled by a steering operation valve 10 disposed between the steering cylinder 6 and the steering pump 2. When a steering spool 12 is rotated with a steering handle 11, so as to open the meter-in opening of the steering operation valve 10 communicating with the steering cylinder 6, the steering cylinder 6 is supplied with oil, discharged from the steering pump 2, through the steering spool 12 and a feedback motor 13, to drive the steering system of the work vehicle. When a steering sleeve 14 is rotated in the same direction as the steering spool 12 by revolving the feedback motor 13 and is located at the same position as the steering spool 12, the supply of oil to the steering cylinder 6 is stopped.
Here, if the steering operation valve 10 is operated, the meter-in opening increases in its opening area to decrease the differential pressure between the front and the rear of the meter-in opening. According to the decrease of the differential pressure between the input pressure and the output pressure of the meter-in opening, the load-sensing valve 4 moves in the direction of the position a, due to the spring force f, to increase the discharge of the steering pump 2. According to the increase in the discharge of the steering pump 2, the differential pressure between the input pressure and the output pressure of the meter-in opening increases to balance with the spring force f of the load-sensing valve 4. In this manner, because the differential pressure between the front and the rear of the meter-in opening is kept constant when the steering valve 10 is operated, the discharge of the steering pump 2, according to the operation speed of the steering operation valve 10, can be obtained.
The engine 1 drives a fixed displacement work-machine pump 9 and a pilot pump 19, which is like the steering pump 2. The fixed displacement work-machine pump 9 drives work-machine cylinders 7 and 8 through work-machine operation valves 17 and 18, independently of the steering pump 2. The pilot pump 19 supplies initial pressures to pilot valves 22 and 23 which generate pilot pressures for operating the work-machine operation valves 17 and 18.
In the second background example (Japanese Unexamined Patent Publication No. 3-186600), oil discharged from a steering pump is made to join in a work-machine hydraulic circuit, so as to be supplied to a work machine, upon operating the work machine. In independently operating a steering system, a variable displacement hydraulic pump therefore discharges an oil quantity required for the steering operation in accordance with the rotational speed of a steering handle regardless of the rate of rotation of an engine, in the range to the maximum discharge of the steering pump, as shown in FIG. 7.
If the work machine is operated at the same time, the discharge of the steering pump becomes the maximum. Oil discharged in proportion to the rate of rotation of the engine is divided preferentially to a steering operation valve in a flow dividing valve and the residual oil is supplied to a work-machine operation valve.
The first background example is a load-sensing type, in which the steering pump 2 merely discharges an oil quantity in accordance with the degree of operation of the steering operation valve 10 regardless of the rate of rotation of the engine 1. It is thus possible to save energy in the middle to high speed range of the engine 1. But in the case of a work vehicle for loading earth and sand, which performs a so-called V-shape operation with switchovers between the forward and backward movements of the vehicle, the engine 1 is controlled at a low speed upon a switchover, to soften the shock due to the switchover and prevent the load from falling out of the bucket. When the rotational speed of the engine 1 is low, the displacement of the steering pump 2 is controlled to the maximum in order to ensure an adequate oil quantity in accordance with the degree of operation of the steering operation valve 10. The variable displacement steering pump thus needs the same pump capacity as a fixed displacement steering pump from the viewpoint that the pump capacity depends upon the oil quantity required when the engine 1 is at a low speed. The variable displacement type pump also causes an increase in cost. Moreover, the work-machine pump 9 employs a large-capacity fixed displacement pump because the steering pump 2 does not assist the work-machine operation valves 17 and 18 with oil. This also causes an increase in cost.
In the second background example, the maximum displacement of the steering pump occurs when the work machine is operated and the steering pump gives a large discharge in proportion to the rate of rotation of the engine. But because the residual oil from the steering operation is supplied to the work-machine operation valve, through a steering gear preference flow-dividing valve, and the discharge of the steering pump can effectively be utilized in a simultaneous operation with the work machine, the total capacity of the steering and work-machine pumps can be decreased in comparison with the first background example so that the cost of both pumps can be decreased. In the case of high oil pressure for the work machine, however, there is a problem in that the work-machine pressure acts on the steering pump and the pressure loss increases upon decreasing the work-machine pressure to the steering pressure.